Toasted banana, Jack!


This is the banana jack housing of the positive 12V cable for my Reprap 3D printer. This is plugged into a PC power supply rated at 475 watts, 380 watts for the 12V rails I am using in parallel.

While doing a 4-hour print I noticed how warm the 120V power supply cable was getting (about 30C). The 12V cable was also warm to the touch, but the banana jacks felt HOT. I decided to watch it, and let the print finish. After it finished and everything cooled, I examined the jacks and found the red one as you see it in the photo.

Before using these cables, which I made in school, I checked that they were appropriate. I found that the most current I could expect was 20 A. I found some evidence that banana jacks could carry up to 20 A. The cables could carry 22 A.

So why did this part get so hot that it blackened and blistered? I’m not entirely sure. I think it was simply too little contact between the wire and the banana jack hardware. When I opened the connector I found that the wire was bunched up instead of nicely wound around the post. If there was poor contact, perhaps it had a higher resistance than the rest of the cable, and the power dissipated was just too much heat for the connector housing to give off through convection and radiation.

Or, maybe this banana jack was simply not rated for the current. The black one didn’t burn out blister, but it was too hot for comfort while printing. And remember I said I found some evidence? I wanted to use the jacks I already had, so I just Googled around until I found something that said 20 amps. Lazy and irresponsible! The right thing would have been to use a data sheet for the actual parts I was using, if I had considered that it is a fire risk. I didn’t have a part number, and it is not wise to assume all jacks were created equal.

For now, I am using tinned wires wrapped and clamped on binding posts. I would like to find a better connector in the future.

Lessons learned? Home made experimental high current devices deserve supervision. And don’t be lazy or cheap in  choosing the right parts for the job, particularly anything high current.

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